synced-tab-dialog: Fix named icons

The computer icon was still dark in dark variant. The placeholder icon was just missing because we've moved the icon in resources at some point. Since that's not what we use for tabs anyway, fix that as well.

While we're here, fix a leak - we were passing ephy_pixbuf_get_from_surface_scaled() results into the tree store and forgetting about them.

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Before

After

Probably makes sense to get into 42 as well given everything.